KL-6 is a sialylated carbohydrate antigen that is involved in pulmonary fibrosis (Non-Patent Literature (NPL) 1, Patent Literature (PTL) 1). KL-6 levels are measured for the diagnosis and determination of therapeutic strategies for interstitial pneumonitis because elevated KL-6 levels and their fluctuation in interstitial pneumonitis indicate a pathological condition (PTL 1). A method of predicting the onset of interstitial pneumonitis caused by interferon administration by measuring serum MUC-1/KL-6 levels (PTL 2), a method of examining prognosis in lung cancer patients by measuring KL-6 (PTL 3), and a method of detecting intraductal papillary mucinous carcinoma or pancreatic cancer by measuring KL-6 in pancreatic juice (PTL 4) have been disclosed. In recent years, the need has increased for KL-6 measurement for the diagnosis and determination of therapeutic strategies for interstitial pneumonitis including drug-induced interstitial pneumonitis, collagen disorder-induced interstitial pneumonitis, etc., diagnosis of patients with cancers of lung, pancreas, etc., and diagnosis and determination of therapeutic strategies for interstitial pneumonitis in patients treated with antibody preparation for rheumatoid arthritis, Crohn's disease, generalized juvenile idiopathic arthritis, Castleman's disease, etc.
In patent literature 1-4, enzyme-linked immunosorbent assay (hereinafter, ELISA method) has been used for KL-6 measurement. Although ELISA is a reliable method, latex immunoagglutination assay is superior for rapid, easy, and cheap testing of a large number of samples and is widely used as a clinical reagent for measuring trace components.
However, when a sample containing the rheumatoid factor is tested by an immunological measurement method such as ELISA or latex immunoagglutination, the occurrence of a nonspecific reaction owing to interference from the rheumatoid factor depending on the sample, is a known problem (NPL 2). In addition, the occurrence of a nonspecific reaction due to interference from heterophile antibodies (anti-mouse immunoglobulin antibody: HAMA etc. and anti-goat immunoglobulin antibody: HAGA etc.) is a known problem.
A method that removes the Fc portion from the antibody bound to the latex, a method that uses an antibody that binds to the rheumatoid factor (PTL 5), and other similar methods are known to inhibit interference from the rheumatoid factor in the latex immunoagglutination assay.
However, the present inventors have found samples in which the occurrence of nonspecific reactions could not be adequately inhibited even by the abovementioned methods.